This one was a hard one to answer because there are a few issues I feel nearly equally strongly about. Health care and reproductive rights were exceptionally close behind my choice of marriage equality.

I believe that any two people should be allowed to bind their lives together on paper without necessarily getting married. In my fantasy-land Utopia, marriage as we know it now is a purely religious and/or spiritual ceremony that has no legal binding whereas a union/partnership/etc. is the document the state cares about. Two consenting adults regardless of their gender/sex/whatever should be allowed to enter a contract with each other, even if they don't love each other romantically.

I feel like "taxes" encompasses many of those issues (including the budget) since it deals with not only what I am paying to the government, but also where my money is being allocated. I care deeply about many of these topics and many of those are supported by my tax dollars. :)